This application is based upon and claims the benefit of priority of Japanese Patent Applications No. H.10-157846 filed on Jun. 5, 1998 and No. H.10-205472 filed on Jul. 21, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a spark plug for internal combustion engine provided with a noble metal chip bonded on a center and/or ground electrode, in particular, a life time improvement of the bonding strength of the chip.
2. Description of Related Art
A spark plug has generally a center electrode fitted through an insulator into a housing and a ground electrode integrated with the housing. The portion of the center electrode exposed out of the end of the insulator faces the ground electrode to form a spark gap within which a spark is discharged. To improve the life time and the performance of the spark plug, a noble metal chip is bonded on the center and/or ground electrode to constitute a spark discharge spot for the spark gap.
Conventionally, a platinum (Pt) alloy has been widely used as material for the noble metal chip. However, the Pt alloy has a demerit that the consumption resistance thereof is presumed not to be sufficient to meet more severe engine specifications for vehicles in future. Therefore, the use of the iridium (Ir) alloy having a melting point higher than that of the Pt alloy has been recently studied and an iridium-rhodium (Irxe2x80x94Rh) alloy and the like have been proposed, as shown in JP-A-9-7733.
In case of the Pt alloy chip, resistance welding has been employed in general for bonding the chip and the center and/or ground electrode in view of easy fabrication and inexpensive cost. However, material of the center and/or ground electrode on which the chip is bonded is usually a nickel (Ni) base alloy and the difference between the linear expansion co-efficient of the Ni alloy and that of the Ir alloy is larger than the difference between that of the Ni alloy and that of the Pt alloy. Therefor, if the spark plug having the Ir alloy chip bonded on the electrode by the resistance welding is used in a high temperature combustion chamber, a great thermal stress produced at the junction of the chip and the electrode due to the above mentioned larger difference of the linear expansion co-efficient may cause a crack or separation of the junction of the chip and the electrode and, as the worst case, the chip may be left out from the electrode.
When the chip made of the Ir alloy is bonded on the electrode, a laser beam welding is considered to be preferable to limit the possible separation of the chip and the electrode during the life time because the chip and the electrode may be sufficiently molten due to the high density of its energy. If an alloy containing Ni and Ir is sufficiently constituted as a molten portion extending from the junction of the chip and the electrode to both sides of the chip and the electrode, the reliable bonding strength of the junction can be secured as the thermal stress may be absorbed and alleviated.
However, the Ni and Ir alloy molten portion tends to be constituted not uniformly but locally so that the bonding strength quality may be lowered. In particular, the laser beam welding of the chip on the ground electrode has an inherent difficulty for effectively bonding all around the periphery of the chip, without bonding a spark discharge surface of the chip, by rotating the ground electrode over the fixed laser beam radiating device because of the L shape construction that the ground electrode extending from the housing surrounding the outer periphery of the center electrode is vended to face a leading end of the center electrode. The laser beam welding of the chip before having bonded the electrode on the housing will also bring the same difficulty.
Further, according to the conventional embodiment as described in U.S. Pat. No. 4,771,210, the ground electrode was provided with a bore penetrating from the surface on the side of the discharge gap to the opposite surface into which the pin-shaped noble metal chip is inserted and is caulked in or welded to the ground electrode on the rear side by laser or argon, to secure the fixing of the chip on the ground electrode. However, it is presumed to be not good at productivity to provide the bore on the electrode because an extreme accuracy is required for fabricating the bore, while the molten portion is not formed on the spark discharge surface of the chip.
The present invention has been made in view of the above mentioned problem, and an object of the present invention is to provide a spark plug for internal combustion engines having an Ir alloy chip bonded on a Ni base alloy center and/or ground electrode by the laser beam welding, in which the molten portion has a reliable bonding strength against the thermal stress.
It is one of the aspect of the present invention to have a construction that the chip is bonded on a surface of the leading end of the ground and/or center electrodes to form a molten portion, but the molten portion is not formed on the spark discharge surface of the chip.
As shown in FIG. 3, the molten portion (70) constituted by an alloy containing Ni and Ir extends from the inside of the chip (52) through the junction (60) of the chip (52) and the electrode (3 or 4) somewhat deep into the electrode (3 or 4). How the molten portion is shaped is important, in particular, how many length the molten portion occupies to the length of the junction of the chip and the electrode and how deep the molten portion is protruded into the electrode are important factors. A ratio of B/A is preferably not less than 0.5, where A is a maximum length from the end (61) of the junction (60) on the side of the center electrode to the end (72) of the molten portion on the junction (60) on the opposite side to the center electrode and, within the length A, B is a length of both ends (71 and 72) of the molten portion.
Further, it is preferable that a ratio of d/t is not less than 2, but not more than 4, where t is a length of the chip (52) perpendicularly extending from the junction (60) and d is a sum of the length t and a length of the molten portion (70) protruding from the junction (60) into the ground electrode (4).
Unless the ratio of B/A is less than 0.5, the separation of the chip from the electrode due to the thermal stress may be prevented. Further, unless the ratio of d/t is less than 2, the separation of the molten portion from the electrode due to the thermal stress may be prevented because the molten portion contains more than 20 weight percent of Ir as an Ir and Ni alloy and, unless the ratio of d/t is more than 4, the separation of the molten portion from the chip due to the thermal stress may be prevented because the molten portion contains less than 80 weight percent of Ir as Ir and Ni alloy.
Another aspect of the present invention is to provide a construction that a longitudinal side surface of the pillar chaped chip is bonded on an end surface, an inner surface or end surface of the leading end of the ground electrode or on an end surface of the center electrode so that the end surface of the chip may constitute a spark discharge surface.
Further aspect of the present invention is to provide a spark plug for internal combustion engines having a relatively small sized noble chip bonded on a ground electrode in which the laser beam is applied from the outer surface of the leading end of the ground electrode to constitute a molten portion extending from the ground electrode into an inside of the chip. The spark plug according to the present invention can be manufactured at the inexpensive cost because the amount of the noble metal to be used is minimized and the bore in the ground electrode as shown in the conventional spark plug mentioned above is not necessary.
To manufacture the above mentioned spark plug, the chip may be fixed tentatively on a surface of the electrode by resistance welding and, then, bonded by laser beam welding so as to constitute a molten portion, which makes the laser beam welding easy.
Further, according to the embodiments of the present invention, as the molten portion, which is not good at the consumption resistance, is not formed on the spark discharge surface of the chip, the longer life and higher performance of the spark plug may be realized.